The present invention relates generally to communication systems, and more particularly, to improved frequency and phase locked loops for acquiring a carrier of a communication signal by measuring both phase and frequency.
Phase locked loops have been used for many years to provide a carrier reference for demodulation of a signal. In particular quadrature amplitude modulated (QAM) signals have used the phase locked loops to generate a carrier to extract the in-phase and quadrature components of a signal to demodulate the data signals that are contained in the signal.
The structure of a typical conventional phase locked loop used in a demodulator is shown in FIG. 1. The phase locked loop shown in FIG. 1 is a second-order phase locked loop that is able to adjust the phase of an input signal to remove any bias in the phase of the signal as it comes from the demodulator.
The conventional phase locked loop can be modeled as an infinite impulse response filter. A control loop measures the error signal, the output of the phase detector, and feeds that error signal back to correct the phase of the carrier used for demodulation. The use of the integrator makes the loop a second order loop with advantages in the operation of the circuit. A text by Floyd Gardner, entitled xe2x80x9cPhase lock Techniques,xe2x80x9d published by Wiley and Sons, NY, 1979 provides for a more complete discussion of phase locked loops.
The integrator forms the integral of the phase, the frequency of the error signal to be used in the correction of the carrier used in the demodulation.
Other forms of phase locked loops also form control loops that control the phase and frequency of a numerically controlled oscillator or voltage controlled oscillator used to lock to the carrier of the signal being demodulated.
The phase locked loop has problems when the initial frequency of the oscillator is not near to the frequency of the signal. The loop must xe2x80x9chuntxe2x80x9d for the frequency of the signal. If the frequency of the signal is too far from the initial frequency of the oscillator, the loop will fail to lock.
The bandwidth of the loop may be increased by decreasing the gain of the loop to increase the acquisition bandwidth of the loop. The increase in the bandwidth allows more noise in the loop, increasing the phase error of the loop and contributing to errors in demodulation of the signal.
It is an objective of the present invention to provide for improved frequency and phase locked loops that acquire a carrier of a communication signal by measuring both phase and frequency.
To meet the above and other objectives, the present invention provides for improved frequency and phase locked loops that are well adapted for use in communication systems. The frequency and phase locked loops are used to acquire a carrier of a communication signal by measuring both phase and frequency.
The present invention provides for the measurement of both frequency and phase error during carrier acquisition of a communication signal. The measurement uses a finite impulse response (FIR) filter as well as the usual feedback loop in the processing. The present invention is able to acquire the carrier signal much more quickly with less phase noise in the demodulation of the signal compared to conventional techniques.